To fabricate high performance optical filters it is necessary to accurately deposit all the layers of the multilayer design with sufficient accuracy to obtain the desired performance. Many high performance optical filters require a large number of layers, sometimes 100 layers or more, with a wide range of thicknesses. Layer thickness monitoring is required to achieve the desired layer thicknesses as close as possible.
Many types of filters, such as short wave band pass filters, long wave band pass filters, and broad band antireflective filters have optimum designs that are not quarter wave in nature. Rather, the best performing designs contain many layers each having different thicknesses.
There is a greater need today for high performance filters in the military and medical communities. There is a need to have an improved method of fabricating such filters. New design techniques in the last two decades have enabled the design of filters having higher performance; such as anti reflection coatings over very wide spectral bands, sharper and cleaner edge filters, and arbitrary spectral response filters. These designs consist of many layers, some thin, and generally all different thicknesses. The advantage of these designs is that better performance is seen. But the disadvantage is that they are more sensitive to thickness errors. Thus they require a more accurate thickness monitor.
Optical monitoring is described in the books by H. A. Macleod, THIN-FILM OPTICAL FILTERS, third edition, Taylor & Francis, 2001; and Ronald R. Willey, PRACTICAL DESIGN AND PRODUCTION OF OPTICAL THIN FILMS, second edition, Marcel Dekker, Inc. 2002. Turning point monitoring is a well known and highly successful technique for depositing designs consisting of multilayer coatings whose thicknesses are all multiples of quarter wave optical thickness. When monitoring the reflectance at the wavelength where the layers are one quarter wave optical thickness of such designs, the monitor exhibits self compensation. This means that thickness errors in one layer will be corrected when the succeeding layers is terminated at its turning point. A turning point occurs when the reflection changes direction, either it goes through a maximum or a minimum. But when the design consists of layers which are not quarter wave in thickness, this self compensation does not operate. Cheng-Chung Lee, et al. in a paper entitled, “Multilayer coatings monitoring using admittance diagram,” Opt. Exp. 16, 6119-6124 (2008), have proposed a monitor that converts the transmittance signal to admittance in real time which then allows compensation to be calculated for non quarter wave designs. However, the method requires very precise transmittance measurements during deposition. The conversion from transmittance to admittance is inherently unstable because the measured transmittance is a real quantity and lacks phase information while admittance is a complex quantity which has two values, amplitude and phase. According to this paper the stability requirements are of the order of 0.1% rms of the monitor signal over long periods of time for the deposition of several layers. Typically 1% would be considered very good for optical monitor signals. Other tight constraints are also necessary. Byung Jin Chun and Chang Kwon Hwangbo in a paper entitled, “Optical monitoring of nonquarterwave layers of dielectric multilayer filters using optical admittance,” Opt. Exp. 14, 2473-2480 (2006), describe a method for determining the error in the previous layer thickness based on small differences in the turning point reflectance of the current layer from that predicted from the design. This error information is then used to calculate a corrected thickness for the current layer and next layer to compensate for the thickness error in the previous layer. Again, this method requires absolute reflection measurements as well as other stringent requirements which are generally not realized in practice. Brian T. Sullivan and J. A. Dobrowolski, in 2 papers entitled, “Deposition error compensation for optical multilayer coatings, I. Theoretical description,” Appl. Opt. 31, 3821-3835 (1992) and “Deposition error compensation for optical multilayer coatings. II. Experimental results-Sputtering system,” Appl. Opt. 32, 2351-2360 (1993), describe another approach that uses a wide band or a multi-wavelength optical monitor to fit all the previously deposited layer thicknesses. These new thickness values are then used to perform a re-design of the remaining layer thicknesses and the deposition proceeds using these new target thicknesses. This reverse engineering approach attempts to solve an inverse problem which is inherently unstable. While it may work in some cases, in general reverse engineering will find a non-unique solution which is not the real values for thicknesses. Without good estimates of the thickness error of the deposited layers one cannot find thicknesses for later compensating layers.
I have developed a method that uses a single wavelength optical monitor which determines the error in the previous layer and recomputes the thicknesses of the currently depositing layer and the next layer to compensate for that error. This new method is more robust than previous methods with respect to noise in the optical monitor signal. This method works for designs that are not necessarily quarter wave in thickness.